This invention relates to an improved method for encapsulating and reinforcing the electrical interconnections between an integrated circuit chip and a substrate. It also relates to an integrated circuit chip assembly produced by said method.
An integrated circuit chip assembly generally comprises an integrated circuit chip attached to a substrate, typically a chip carrier or a circuit board. The most commonly used integrated circuit chip is composed primarily of silicon having a coefficient of thermal expansion of about 2 to 4 ppm/xc2x0 C. The chip carrier or circuit board is typically composed of either a ceramic material having a coefficient of thermal expansion of about 6 ppm/xc2x0 C., or an organic material, possibly reinforced with organic or inorganic particles or fibers, and having a coefficient of thermal expansion in the range of about 6 to 50 ppm/xc2x0 C. One technique well known in the art for attaching the chip to the substrate is flip chip bonding. In flip chip bonding, a pattern of solder balls, usually having a diameter of about 0.002 to 0.006 inches, is formed on one surface of the integrated circuit chip, fully or partially populating the active chip surface with interconnection sites. A matching footprint of solder wettable terminals is provided on the substrate. The integrated circuit chip is then placed in alignment with the substrate, and the chip to substrate connections are formed by reflowing the solder balls. During operation of an integrated circuit chip assembly, cyclic temperature excursions cause the substrate and the integrated circuit chip to expand and contract. Since the substrate and the integrated circuit chip have different coefficients of thermal expansion, they expand and contract at different rates causing the solder ball connections to weaken or even crack as a result of fatigue. To remedy this situation, it is common industry practice to reinforce the solder ball connections with a thermally curable polymer material known in the art as an underfill encapsulant.
Underfill encapsulants are typically filled with ceramic particles to control their rheology in the uncured state, and to improve their thermal and mechanical properties in the cured state. Underfill encapsulants have been used widely to improve the fatigue life of integrated circuit chip assemblies consisting of an integrated circuit chip of the flip chip variety attached to a substrate comprised of an alumina ceramic material having a coefficient of thermal expansion of about 6 ppm/xc2x0 C. More recently, integrated circuit chip assemblies have been manufactured using substrates comprised of a reinforced organic material having a composite coefficient of thermal expansion of about 20 ppm/xc2x0 C.
The underfill encapsulation process is typically accomplished by dispensing a liquid encapsulant directly onto the substrate at one or more points along the periphery of the integrated circuit chip. The encapsulant is drawn into the space between the integrated circuit chip and the substrate by capillary forces, and forms a fillet around the perimeter of the integrated circuit chip. The diameter of the filler particles in the encapsulant is typically smaller than the height of the space so that flow is not restricted, with typical encapsulants having viscosities of about 10 Pa-s at the dispense temperature. Once the underfilling process is completed, the encapsulant is heat cured in an oven. Cured encapsulants typically have coefficients of thermal expansion in the range of 20 to 40 ppm/xc2x0 C. and a Young""s Modulus of about 1 to 3 Gpa, depending on the filler content and the type of chemistry. Depending on the materials the integrated circuit chip and the substrate are composed of, it may be desirable to further alter the cured properties of the encapsulant. However, the requirement that the encapsulant have low viscosity in the uncured state so that it flows readily into the space between the integrated circuit chip and the substrate severely restricts the formulation options. For example, the addition of more ceramic filler would result in a lower coefficient of thermal expansion, but would cause an increase in the viscosity of the uncured encapsulant. Furthermore, even with the use of underfill encapsulation, fatigue life of an integrated circuit chip assembly is shorter when the integrated circuit chip is interconnected to an organic substrate as opposed to a ceramic substrate due to the greater mismatch in thermal expansion between the typical integrated circuit chip and organic substrates.
Also known in the art is a method wherein a package body is formed around the perimeter of the flip chip using a two step process. First the integrated circuit chip assembly is underfilled as described above. Next, a package body is formed around the perimeter of the integrated circuit chip using a molding process.
The prior art also suggests a process wherein additional reinforcement is achieved by forming a package body around the integrated circuit chip assembly using a single step operation. In this process, a large opening of about 50% of the size of the integrated circuit chip is formed in the substrate under the integrated circuit chip. This approach essentially eliminates the space between the integrated circuit chip and the substrate that is typical of a conventional integrated circuit chip to substrate interconnection, but has the drawback of limiting the active surface area of the integrated circuit chip that can be utilized for forming interconnections because only the perimeter of the active surface of the integrated circuit chip can be used.
It is an object of the present invention to provide a method of encapsulating and reinforcing the electrical interconnections of an integrated circuit chip assembly which allows the use of highly viscous encapsulating materials and eliminates the need to use different encapsulating materials for underfilling and overmolding. It is also an object of this invention to provide a method of encapsulating an integrated circuit chip assembly which enables simultaneous underfilling and overmolding without reducing the active interconnection area of the integrated circuit chip or substantially altering the substrate design. Another object of this invention is to provide an integrated circuit chip assembly having a reinforced electrical interconnection which is more resistant to weakening as a result of stress created by the differences in coefficient of thermal expansion between the integrated circuit chip and the substrate.
This invention provides an improved method for encapsulating the solder ball interconnections of an integrated circuit assembly which accommodates the use of high viscosity encapsulating materials and allows simultaneous underfilling and overmolding; and eliminates the need for a dam to contain flow. In accordance with the preferred embodiment of this invention, an integrated circuit chip assembly comprised of an integrated circuit chip mounted on a chip carrier or directly on a circuit board in a standoff relationship by solder ball connections is provided.
A mold is placed over the integrated circuit chip. The mold is constructed with an opening extending from the inside surface of the mold to the outside surface of the mold and at least one vent. External pressure is applied to the mold to seal the mold to the surface of the substrate to which the integrated circuit chip is attached. The mold is constructed so that there is a space between the inner surface of the mold and the integrated circuit chip. A metered volume of encapsulant material is dispensed through the opening into the space surrounding the integrated circuit chip and the space between the integrated circuit chip and the chip carrier or circuit board. The preferred encapsulant material comprises a high strength thermosetting one part epoxy containing about 50% to 80% by weight of inorganic electrically non-conductive filler and has a viscosity at 25xc2x0 C. of about 250 Pascal-seconds measured using a Brookfield viscometer, model HBT, with a CP-52 cone head, at 2 rpm; although materials having viscosities in the range of about 10 to 1,000 Pascal-seconds may also be used.
After the required amount of encapsulant material is dispensed, the material is cured to form a bond between the integrated circuit chip and the chip carrier or circuit board and reinforce the solder ball connections.